According to the state of the art suction devices are e.g. used to grip sheets of paper or items consisting of several layers of paper and to separate them from a stack. Such suction devices consist of an elastically deformable sucker which is arranged at the end of a suction line. For separating the gripped item from the stack, this end of a suction line is movable relative to the stack or the sucker is movable relative to the end of the suction line. The sucker is thus lowered onto the stack, evacuated and lifted off the stack together with the gripped uppermost item. Hereby the relative positions of the stack and the sucker in a lowered position as well as the coordination of the movement of the sucker, the evacuation and the strength of the suction force produced by the vacuum must be very carefully adjusted to suit each other. This is especially the case if the items to be gripped are very lightweight and not fully tight and if it is important that only one item is gripped at a time. This however is often not possible with a sufficient security for items which are difficult to handle even when things are very well matched.
The publication DE-19603040 describes a suction device which works reliably in the most varied applications. This means in other words that e.g. when using the described suction device, with high reliability only one product is lifted from a stack in each cycle. This suction device is characterized in that it carries out a pre-travel combined with a pre-depression by the combined effect of which the product to be lifted can easily be separated from the stack or from a corresponding support and is then actually lifted. Pre-stroke and predepression precede a stroke and an end-evacuation and they are forcibly coupled. Hereby the pre-depression is higher (nearer to ambient pressure) than the end-evacuation. The pre-depression is such that it is just sufficient to separate an area of the product to be moved from a stack or support and that it is not sufficient for the full movement of the product. By using the pre-travel with the less forceful pre-depression it is possible to reduce the probability of gripping more than one product and it becomes possible to extend the application range of suction devices to softer, more lightweight and less tight products.